The invention concerns imprint templates for use in nanoimprint lithography methods, a nanoimprint device suitable for UV nanoimprint lithography methods and a nanostructuring method for direct structuring of a UV-sensitive imprint material on a substrate.
Nanoimprint lithography methods are mechanical direct structuring methods in which an embossing die, also called an embossing punch, imprint mask or hereinafter imprint template, having relief on its surface, is forced with its relief into a material being structured, the so-called imprint material, in order to deform and structure it in this way. As a result, the relief is imaged one to one as a negative in the imprint material.
One then distinguishes essentially nanostructuring methods that use high temperatures and pressures in order to deform thermoplastic materials, so-called thermal nanoimprint lithography or T-NIL, from methods that use ultraviolet light (UV light) in order to cure UV-curing polymers in contact with the embossing die and to form the die structure, so-called UV nanoimprint lithography or UV-NIL. Different combinations of the two mentioned Methods are also possible.
Since nanoimprint lithography methods are structuring methods in which the desired nanostructures can be generated directly with high precision at comparatively low cost, they are an excellent alternative for appropriate applications to traditional photo and electron lithography techniques that are more expensive by orders of magnitude, in which the secondary effects of diffraction and scattering of the particles used there for exposure and therefore structuring must also be minimized in very demanding fashion.
In previous nanoimprint devices either fully nontransparent imprint templates have been used for T-NIL (typically silicon templates) or fully transparent imprint templates for UV-NIL (typically quartz templates). The transparent UV-NIL templates can also be used in principle for T-NIL.
For nontransparent T-NIL templates structure, nontransparent silicon chips that contain the negative of the structures that are to be imprinted on the imprint material situated on the substrate are ordinarily fastened to nontransparent substrates.
For transparent UV-NIL templates two essential variants are found in the prior art: use of monolithic imprint templates or use of combined imprint templates consisting of a completely unstructured, transparent substrate to which a transparent chip is fastened.
Monolithic transparent imprint templates, which generally have a format of 65 mm×65 mm×6.35 mm (W×L×H), contain a protruding structure block, the so-called mesa in the center of the template. This mesa rises several micrometers, usually up to 50 μm over the remaining template surface. The dimensions (or lenghths) of the side edges of the mesa are generally smaller than the 65 mm lengths of the corresponding side edges of the entire template; the usual side edge lengths of the mesa generally being less than 25 mm. The useful structure that functions as imprint structure is situated on the mesa. Structuring of the mesa can occur by standard methods of semiconductor technology, generally by anisotropic wet or dry etching methods after photolithographic generation of a corresponding etching mask. However, mechanical methods, like milling, are also possible structuring methods of the mesa. The structure on this mesa corresponds to the negative of the structure that is to be imprinted on the imprint material situated on the substrate.
In a combined transparent imprint template, on the other hand, a completely unstructured transparent substrate is involved whose dimensions are ordinarily 65 mm×65 mm×6.35 mm and a transparent chip fastened to it, which is usually less than 1 mm thick, generally consists of quartz and is generally fastened to the unstructured transparent substrate. Here again the structure on the chip corresponds to the negative of the structure that is to be imprinted in the imprint material situated on the substrate.
According to the prior art, nanoimprint devices are constructed so that the substrate with the layer to be structured situated on it, which is also referred to as imprint material, lies on a support, the so-called chuck, which is mechanically and thermally stable and can consist, for example, of silicon carbide. A holder is situated above this substrate, but still not in contact with it before performance of the nanoimprint lithography method, which can be preferably manipulated in the x-, y- and z-direction, which contains the imprint template whose structured part points toward the layer of the substrate being structured.
Ordinarily in the UV-NIL technique, the UV radiation source for exposure of the UV-sensitive imprint material is directly integrated in the holder. The holder for this UV-NIL technique is transparent, like the imprint template and is exposed through the entire imprint template.
The requirement for transparency of the imprint template together with the requirements for chemical and mechanical properties of an imprint template like high hardness and chemical stability, however, leads to a very restricted selection of materials for transparent imprint templates. The most used material here is quartz. Although the manufacturing technique for quartz imprint templates can be referred to as advanced in comparison with other possible materials, structuring of quartz represents a greater challenge and is mastered in the structure size required here only by a few specialized suppliers. The development of corresponding competence for quartz structures is very time-consuming and cost-intensive and is therefore generally not effective for users of nanoimprint lithography methods. Both the monolithic transparent UV-NIL templates and the structured quartz chips used for combined transparent UV-NIL templates, which are then fastened to substrates, must therefore be acquired cost-intensively from the specialized suppliers.
Additional problems in using such completely transparent imprint templates occur, if one intends to use these imprint template also for thermal nanoimprint lithography methods or for a combination of both methods:
Because of the increased hazard of damage during thermal nanoimprint lithography methods it is all the more so important here to be able to produce cost-effective imprint templates. The now employed transparent imprint templates also have thermal expansion coefficients that generally deviate sharply from the thermal expansion coefficients of the now employed substrates, like silicon, on which the structure is to be imprinted. Large differences in thermal expansion coefficients of the imprint template and substrate rule out use of these templates in thermal nanoimprint lithography methods or combined UV and thermal nanoimprint lithography methods for very demanding applications with high resolution.
In order to be able to utilize the advantages of nontransparent imprint templates for UV-NIL a further possibility of UV-NIL is therefore described, in which the nontransparent imprint templates are used: For this purpose, however, the transparent substrate is exposed from below. This means that the UV source must be situated beneath or in the chuck and both this chuck and the substrate must necessarily be transparent. Such an arrangement, however, entails some problems: The chuck should simultaneously offer possibilities for heating or cooling but a UV source situated in it would only make this feasible with considerable difficulty and cost.
DE 20 2006 008 399 U1 also describes a variant for production of an at least partially transparent imprint template in which the imprint structure of the imprint template is initially structured in a preoxidation state in order to be able to use simple and well mastered structuring methods for this purpose and the structuring is then fully or partially oxidized in order to make this imprint template transparent if UV nanoimprint lithography is to be used. In the preoxidation state, if a silicon-containing imprint template is involved, not pure silicon is at issue, but a nonstoichiometric SiOx compound with on average 1<x<2. During use of other materials nonstoichiometric compounds are also used, which must then be structured accordingly and whose final quality depends on the corresponding preoxidation state and final treatment by oxidation.